The bulk of the document is written in LaTeX, and it is possible that they used the listings package to write the code samples. See section 2.10 of The Listings Package document for variable column code samples. In more regular code snippets, the results can be very nice. http://mirror.ox.ac.uk/sites/ctan.org/macros/latex/contrib/l...
It actually works quite well if you pick the right font. Stroustrup uses it in The C++ Programming Language and it is easier to read.